Three-dimensional ferroelectric memory array including integrated gate selectors and methods of forming the same

ABSTRACT

A ferroelectric field effect transistor (FeFET) includes a semiconductor channel, a source region contacting one end of the semiconductor channel, a drain region contacting a second end of the semiconductor channel, a gate electrode, a ferroelectric gate dielectric layer located between the semiconductor channel and the gate electrode, and a bidirectional selector material layer located between the gate electrode and the ferroelectric gate dielectric layer.

FIELD

The present disclosure relates generally to the field of semiconductordevices, and particularly to a three-dimensional ferroelectric memoryarray including integrated gate selectors and methods of manufacturingthe same.

BACKGROUND

A ferroelectric material refers to a material that displays spontaneouspolarization of electrical charges in the absence of an applied electricfield. The net polarization P of electrical charges within theferroelectric material is non-zero in the minimum energy state. Thus,spontaneous ferroelectric polarization of the material occurs, and theferroelectric material accumulates surfaces charges of opposite polaritytypes on two opposing surfaces. Polarization P of a ferroelectricmaterial as a function of an applied voltage V thereacross displayshysteresis. The product of the remanent polarization and the coercivefield of a ferroelectric material is a metric for characterizingeffectiveness of the ferroelectric material.

A ferroelectric memory device is a memory device containing theferroelectric material which is used to store information. Theferroelectric material acts as the memory material of the memory device.The dipole moment of the ferroelectric material is programmed in twodifferent orientations (e.g., “up” or “down” polarization positionsbased on atom positions, such as oxygen and/or metal atom positions, inthe crystal lattice) depending on the polarity of the applied electricfield to the ferroelectric material to store information in theferroelectric material. The different orientations of the dipole momentof the ferroelectric material may be detected by the electric fieldgenerated by the dipole moment of the ferroelectric material. Forexample, the orientation of the dipole moment may be detected bymeasuring electrical current passing through a semiconductor channelprovided adjacent to the ferroelectric material in a field effecttransistor ferroelectric memory device.

SUMMARY

According to an aspect of the present disclosure, a ferroelectric fieldeffect transistor (FeFET) includes a semiconductor channel, a sourceregion contacting one end of the semiconductor channel, a drain regioncontacting a second end of the semiconductor channel, a gate electrode,a ferroelectric gate dielectric layer located between the semiconductorchannel and the gate electrode, and a bidirectional selector materiallayer located between the gate electrode and the ferroelectric gatedielectric layer.

According to another aspect of the present disclosure, a method offorming a semiconductor device is provided, which comprises: formingchannel-level insulating layers over a substrate; forming a verticalstack of semiconductor channels at levels of the channel-levelinsulating layers; forming a stack including, from one side to another,a ferroelectric gate dielectric layer, a bidirectional selector materiallayer, and a gate electrode directly on the vertical stack ofsemiconductor channels; and providing a source region on one end of eachof the semiconductor channels and a drain region on another end of eachof the semiconductor channels, wherein each of the semiconductorchannels, the source regions, and the drain regions contacts theferroelectric gate dielectric layer, and each of the channel-levelinsulating layers contacts a respective one of the source regions and arespective one of the drain regions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic vertical cross-sectional view of a first exemplarystructure after formation of at least one peripheral device and asemiconductor material layer according to a first embodiment of thepresent disclosure.

FIG. 2 is a schematic vertical cross-sectional view of the firstexemplary structure after formation of an alternating stack ofinsulating layers and sacrificial material layers according to the firstembodiment of the present disclosure.

FIG. 3 is a schematic vertical cross-sectional view of the firstexemplary structure after formation of stepped terraces and aretro-stepped dielectric material portion according to the firstembodiment of the present disclosure.

FIG. 4A is a schematic vertical cross-sectional view of the firstexemplary structure after formation of memory openings and supportopenings according to the first embodiment of the present disclosure.

FIG. 4B is a top-down view of the first exemplary structure of FIG. 4A.The vertical plane A-A′ is the plane of the cross-section for FIG. 4A.

FIG. 5 is a schematic vertical cross-sectional view of the firstexemplary structure after laterally recessing channel-level insulatinglayers selective to inter-transistor-level insulating layers andsacrificial material layers according to the first embodiment of thepresent disclosure.

FIG. 6 is a schematic vertical cross-sectional view of the firstexemplary structure after formation of semiconductor channels having atubular configuration according to the first embodiment of the presentdisclosure.

FIG. 7A is a schematic vertical cross-sectional view of the firstexemplary structure after formation of a layer stack including, from oneside to another, a gate dielectric layer, a selector material layer, anda word line in each memory opening according to the first embodiment ofthe present disclosure.

FIG. 7B is a top-down view of the first exemplary structure of FIG. 7A.The vertical plane A-A′ is the plane of the cross-section for FIG. 7A.

FIG. 8A is a schematic vertical cross-sectional view of the firstexemplary structure after formation of a contact-level dielectric layerand backside trenches according to the first embodiment of the presentdisclosure.

FIG. 8B is a partial see-through top-down view of the first exemplarystructure of FIG. 8A. The vertical plane A-A′ is the plane of theschematic vertical cross-sectional view of FIG. 8A.

FIG. 9 is a schematic vertical cross-sectional view of the firstexemplary structure after formation of backside recesses according tothe first embodiment of the present disclosure.

FIG. 10 is a schematic vertical cross-sectional view of the firstexemplary structure after formation of doped semiconductor layersaccording to the first embodiment of the present disclosure.

FIG. 11 is a schematic vertical cross-sectional view of the firstexemplary structure after lateral recessing of the doped semiconductorlayers according to the first embodiment of the present disclosure.

FIG. 12 is a schematic vertical cross-sectional view of the firstexemplary structure after conformal deposition of a metallic materialaccording to the first embodiment of the present disclosure.

FIG. 13A is a schematic vertical cross-sectional view of the firstexemplary structure after removal of the metallic material from thebackside trenches and from above the contact-level dielectric layeraccording to the first embodiment of the present disclosure.

FIG. 13B is a partial see-through top-down view of the first exemplarystructure of FIG. 13A. The vertical plane A-A′ is the plane of theschematic vertical cross-sectional view of FIG. 13A.

FIG. 14 is a schematic vertical cross-sectional view of the firstexemplary structure after formation of dielectric trench fill structuresaccording to the first embodiment of the present disclosure.

FIG. 15A is a schematic vertical cross-sectional view of the firstexemplary structure after formation of contact via structures accordingto the first embodiment of the present disclosure.

FIG. 15B is a top-down view of the first exemplary structure of FIG.15A. The vertical plane A-A′ is the plane of the schematic verticalcross-sectional view of FIG. 15A.

FIG. 16 is a schematic vertical cross-sectional view of a secondexemplary structure after formation of insulating layers, dopedsemiconductor layers, and sacrificial material layers according to asecond embodiment of the present disclosure.

FIG. 17 is a schematic vertical cross-sectional view of the secondexemplary structure after formation of stepped terraces and aretro-stepped dielectric material portion according to the secondembodiment of the present disclosure.

FIG. 18 is a schematic vertical cross-sectional view of the secondexemplary structure after formation of memory openings and supportopenings according to the second embodiment of the present disclosure.

FIG. 19 is a schematic vertical cross-sectional view of the secondexemplary structure after formation of semiconductor channels having atubular configuration according to the second embodiment of the presentdisclosure.

FIG. 20 is a schematic vertical cross-sectional view of the secondexemplary structure after formation of a layer stack including, from oneside to another, a gate dielectric layer, a selector material layer, anda word line in each memory opening according to the second embodiment ofthe present disclosure.

FIG. 21 is a schematic vertical cross-sectional view of the secondexemplary structure after formation of a contact-level dielectric layerand backside trenches according to the second embodiment of the presentdisclosure.

FIG. 22 is a schematic vertical cross-sectional view of the secondexemplary structure after formation of backside recesses according tothe second embodiment of the present disclosure.

FIG. 23 is a schematic vertical cross-sectional view of the secondexemplary structure after formation of metallic material layersaccording to the second embodiment of the present disclosure.

FIG. 24 is a schematic vertical cross-sectional view of the secondexemplary structure after formation of dielectric trench fill structuresand contact via structures according to the second embodiment of thepresent disclosure.

FIG. 25 is a schematic circuit diagram of the three-dimensional array ofmemory elements according to embodiments of the present disclosure.

DETAILED DESCRIPTION

As discussed above, the present disclosure is directed to athree-dimensional ferroelectric memory array including integrated gateselectors and methods of manufacturing the same, the various aspects ofwhich are described below.

The drawings are not drawn to scale. Multiple instances of an elementmay be duplicated where a single instance of the element is illustrated,unless absence of duplication of elements is expressly described orclearly indicated otherwise. Ordinals such as “first,” “second,” and“third” are employed merely to identify similar elements, and differentordinals may be employed across the specification and the claims of theinstant disclosure. The same reference numerals refer to the sameelement or similar element. Unless otherwise indicated, elements havingthe same reference numerals are presumed to have the same compositionand the same function. Unless otherwise indicated, a “contact” betweenelements refers to a direct contact between elements that provides anedge or a surface shared by the elements. As used herein, a firstelement located “on” a second element may be located on the exteriorside of a surface of the second element or on the interior side of thesecond element. As used herein, a first element is located “directly on”a second element if there exist a physical contact between a surface ofthe first element and a surface of the second element. As used herein, afirst element is “electrically connected to” a second element if thereexists a conductive path consisting of at least one conductive materialbetween the first element and the second element. As used herein, a“prototype” structure or an “in-process” structure refers to a transientstructure that is subsequently modified in the shape or composition ofat least one component therein.

As used herein, a “layer” refers to a material portion including aregion having a thickness. A layer may extend over the entirety of anunderlying or overlying structure, or may have an extent less than theextent of an underlying or overlying structure. Further, a layer may bea region of a homogeneous or inhomogeneous continuous structure that hasa thickness less than the thickness of the continuous structure. Forexample, a layer may be located between any pair of horizontal planesbetween, or at, a top surface and a bottom surface of the continuousstructure. A layer may extend horizontally, vertically, and/or along atapered surface. A substrate may be a layer, may include one or morelayers therein, or may have one or more layer thereupon, thereabove,and/or therebelow.

As used herein, a first surface and a second surface are “verticallycoincident” with each other if the second surface overlies or underliesthe first surface and there exists a vertical plane or a substantiallyvertical plane that includes the first surface and the second surface. Asubstantially vertical plane is a plane that extends straight along adirection that deviates from a vertical direction by an angle less than5 degrees. A vertical plane or a substantially vertical plane isstraight along a vertical direction or a substantially verticaldirection, and may, or may not, include a curvature along a directionthat is perpendicular to the vertical direction or the substantiallyvertical direction.

A monolithic three-dimensional memory array is a memory array in whichmultiple memory levels are formed above a single substrate, such as asemiconductor wafer, with no intervening substrates. The term“monolithic” means that layers of each level of the array are directlydeposited on the layers of each underlying level of the array. Incontrast, two dimensional arrays may be formed separately and thenpackaged together to form a non-monolithic memory device. For example,non-monolithic stacked memories have been constructed by forming memorylevels on separate substrates and vertically stacking the memory levels,as described in U.S. Pat. No. 5,915,167 titled “Three-dimensionalStructure Memory.” The substrates may be thinned or removed from thememory levels before bonding, but as the memory levels are initiallyformed over separate substrates, such memories are not true monolithicthree-dimensional memory arrays.

Generally, a semiconductor package (or a “package”) refers to a unitsemiconductor device that may be attached to a circuit board through aset of pins or solder balls. A semiconductor package may include asemiconductor chip (or a “chip”) or a plurality of semiconductor chipsthat are bonded thereamongst, for example, by flip-chip bonding oranother chip-to-chip bonding. A package or a chip may include a singlesemiconductor die (or a “die”) or a plurality of semiconductor dies. Adie is the smallest unit that may independently execute externalcommands or report status. Typically, a package or a chip with multipledies is capable of simultaneously executing as many number of externalcommands as the total number of planes therein. Each die includes one ormore planes. Identical concurrent operations may be executed in eachplane within a same die, although there may be some restrictions. Incase a die is a memory die, i.e., a die including memory elements,concurrent read operations, concurrent write operations, or concurrenterase operations may be performed in each plane within a same memorydie. In a memory die, each plane contains a number of memory blocks (or“blocks”), which are the smallest unit that may be erased by in a singleerase operation. Each memory block contains a number of pages, which arethe smallest units that may be selected for programming. A page is alsothe smallest unit that may be selected to a read operation.

Referring to FIG. 1, a first exemplary structure according to the firstembodiment of the present disclosure is illustrated. The first exemplarystructure includes a substrate 9 which may be a semiconductor substrate.The substrate may include a substrate semiconductor layer 9 and anoptional semiconductor material layer. The substrate semiconductor layer9 maybe a semiconductor wafer or a semiconductor material layer, and mayinclude at least one elemental semiconductor material (e.g., singlecrystal silicon wafer or layer), at least one III-V compoundsemiconductor material, at least one II-VI compound semiconductormaterial, at least one organic semiconductor material, or othersemiconductor materials known in the art. The substrate may have a majorsurface 7, which may be, for example, a topmost surface of the substratesemiconductor layer 9. The major surface 7 may be a semiconductorsurface. In one embodiment, the major surface 7 may be a singlecrystalline semiconductor surface, such as a single crystallinesemiconductor surface.

As used herein, a “semiconducting material” refers to a material havingelectrical conductivity in the range from 1.0×10⁻⁵ S/m to 1.0×10⁵ S/m.As used herein, a “semiconductor material” refers to a material havingelectrical conductivity in the range from 1.0×10⁻⁵ S/m to 1.0 S/m in theabsence of electrical dopants therein, and is capable of producing adoped material having electrical conductivity in a range from 1.0 S/m to1.0×10⁵ S/m upon suitable doping with an electrical dopant. As usedherein, an “electrical dopant” refers to a p-type dopant that adds ahole to a valence band within a band structure, or an n-type dopant thatadds an electron to a conduction band within a band structure. As usedherein, a “conductive material” refers to a material having electricalconductivity greater than 1.0×10⁵ S/m. As used herein, an “insulatormaterial” or a “dielectric material” refers to a material havingelectrical conductivity less than 1.0×10⁻⁵ S/m. As used herein, a“heavily doped semiconductor material” refers to a semiconductormaterial that is doped with electrical dopant at a sufficiently highatomic concentration to become a conductive material either as formed asa crystalline material or if converted into a crystalline materialthrough an anneal process (for example, from an initial amorphousstate), i.e., to have electrical conductivity greater than 1.0×10⁵ S/m.A “doped semiconductor material” may be a heavily doped semiconductormaterial, or may be a semiconductor material that includes electricaldopants (i.e., p-type dopants and/or n-type dopants) at a concentrationthat provides electrical conductivity in the range from 1.0×10⁻⁵ S/m to1.0×10⁵ S/m. An “intrinsic semiconductor material” refers to asemiconductor material that is not doped with electrical dopants. Thus,a semiconductor material may be semiconducting or conductive, and may bean intrinsic semiconductor material or a doped semiconductor material. Adoped semiconductor material may be semiconducting or conductivedepending on the atomic concentration of electrical dopants therein. Asused herein, a “metallic material” refers to a conductive materialincluding at least one metallic element therein. All measurements forelectrical conductivities are made at the standard condition.

In one embodiment, at least one semiconductor device 700 for aperipheral circuitry may be formed on a portion of the substratesemiconductor layer 9. The at least one semiconductor device mayinclude, for example, field effect transistors. If the at least onesemiconductor device is employed for the peripheral circuitry, the atleast one semiconductor device is herein referred to at least oneperipheral device. For example, at least one shallow trench isolationstructure 720 may be formed by etching portions of the substratesemiconductor layer 9 and depositing a dielectric material therein. Agate dielectric, at least one gate conductor layer, and a gate capdielectric layer may be formed over the substrate semiconductor layer 9,and may be subsequently patterned to form at least one gate structure(750, 752, 754, 758), each of which may include a gate dielectric 750, agate electrode (752, 754), and a gate cap dielectric 758. The gateelectrode (752, 754) may include a stack of a first gate electrodeportion 752 and a second gate electrode portion 754. At least one gatespacer 756 may be formed around the at least one gate structure (750,752, 754, 758) by depositing and anisotropically etching a dielectricliner. Active regions 730 may be formed in upper portions of thesubstrate semiconductor layer 9, for example, by introducing electricaldopants employing the at least one gate structure (750, 752, 754, 758)as masking structures. Additional masks may be employed as needed. Theactive region 730 may include source regions and drain regions of fieldeffect transistors. A first dielectric liner 761 and a second dielectricliner 762 may be optionally formed. Each of the first and seconddielectric liners (761, 762) may comprise a silicon oxide layer, asilicon nitride layer, and/or a dielectric metal oxide layer. As usedherein, silicon oxide includes silicon dioxide as well asnon-stoichiometric silicon oxides having more or less than two oxygenatoms for each silicon atoms. Silicon dioxide is preferred. In anillustrative example, the first dielectric liner 761 may be a siliconoxide layer, and the second dielectric liner 762 may be a siliconnitride layer. The least one semiconductor device for the peripheralcircuitry may contain a driver circuit for memory devices to besubsequently formed.

A dielectric material such as silicon oxide may be deposited over the atleast one semiconductor device, and may be subsequently planarized toform a planarization dielectric layer 770. In one embodiment theplanarized top surface of the planarization dielectric layer 770 may becoplanar with a top surface of the dielectric liners (761, 762).Subsequently, the planarization dielectric layer 770 and the dielectricliners (761, 762) may be removed from an area to physically expose a topsurface of the substrate semiconductor layer 9. As used herein, asurface is “physically exposed” if the surface is in physical contactwith vacuum, or a gas phase material (such as air).

A bottom insulating layer 31 may be formed over the top surface 7 of thesubstrate 9 prior to, during, or after, formation of the at least onesemiconductor device 700 by deposition of an insulating material, suchas silicon oxide.

The region (i.e., area) of the at least one semiconductor device 700 isherein referred to as a peripheral device region 200. The region inwhich a memory array is subsequently formed is herein referred to as amemory array region 100. A staircase region 300 for subsequently formingstepped terraces of electrically conductive layers may be providedbetween the memory array region 100 and the peripheral device region200. In an alternative embodiment, the at least one semiconductor device700 is formed under the memory array region 100 in a CMOS under array(“CUA”) configuration. In this case, the peripheral device region 200may be omitted or used in combination with the CUA configuration. Inanother alternative embodiment, the at least one semiconductor device700 may be formed on a separate substrate and then bonded to substrate 9containing the memory array region 100.

Referring to FIG. 2, a sequence of material layers including two or morerepetitions of a unit layer stack (42, 32A, 42, 32B) can be formed overthe bottom insulating layer 31. The unit layer stack can include, frombottom to top, a first instance of a sacrificial material layer 42, achannel-level insulating layer 32A, a second instance of the sacrificialmaterial layer 42, and an inter-transistor-level insulating layer 32B.The channel-level insulating layer 32A and the inter-transistor-levelinsulating layer 32B are herein collectively referred to as insulatinglayers 32.

An alternating plurality of insulating layers 32 and sacrificialmaterial layers 42 is thus provided. As used herein, an alternatingplurality of first elements and second elements refers to a structure inwhich instances of the first elements and instances of the secondelements alternate. In this case, the insulating layers 32 and thesacrificial material layers 42 alternate within the alternatingplurality of the insulating layers 32 and the sacrificial materiallayers. The alternating plurality of insulating layers 32 andsacrificial material layers 42 is herein referred to as an alternatingstack (32, 42).

The channel-level insulating layers 32A include a first insulatingmaterial. In one embodiment, the first insulating material can include adoped silicate glass material such as borosilicate glass (BSG),borophosphosilicate glass (BPSG), or fluorosilicate glass, or anorganosilicate glass material. The first insulating material can have ahigher etch rate in 100:1 dilute hydrofluoric acid than undoped silicateglass. In one embodiment, the composition of the doped silicate glassmaterial or the organosilicate glass material in the channel-levelinsulating layers 32A can have an etch rate in 100:1 dilute hydrofluoricacid that is greater than the etch rate of undoped silicate glass in100:1 dilute hydrofluoric acid by a factor of at least 3, and/or atleast 10. The thickness of each channel-level insulating layer 32A canbe in a range from 10 nm to 60 nm, although lesser and greaterthicknesses can also be employed.

The inter-transistor-level insulating layers 32B include a secondinsulating material. In one embodiment, the second insulating materialcan include a silicate glass material having a different compositionthan the dielectric material of the channel-level insulating layers 32A.In one embodiment, the second insulating material can include undopedsilicate glass (i.e., undoped silicon oxide, such as silicon dioxide).In one embodiment, the first insulating material of the channel-levelinsulating layers 32A can have an etch rate in a 100:1 dilutehydrofluoric acid solution at 20 degrees Celsius that is at least threetimes an etch rate of the silicate glass material of theinter-transistor-level insulating layers 32B in the 100:1 dilutehydrofluoric acid solution at 20 degrees Celsius. The thickness of eachinter-transistor-level insulating layer 32B can be in a range from 10 nmto 60 nm, although lesser and greater thicknesses can also be employed.

The sacrificial material layers 42 include a sacrificial material thatcan be removed selective to the materials of the insulating layers 32.Generally, the material of the sacrificial material layers 42 is asacrificial material that may be removed selective to the material ofthe channel-level insulating layers 32A and selective to the material ofthe inter-transistor-level insulating layers 32B. As used herein, aremoval of a first material is “selective to” a second material if theremoval process removes the first material at a rate that is at leasttwice the rate of removal of the second material. The ratio of the rateof removal of the first material to the rate of removal of the secondmaterial is herein referred to as a “selectivity” of the removal processfor the first material with respect to the second material. For example,the sacrificial material layers 42 can include silicon nitride,amorphous silicon, polysilicon, or a silicon-germanium alloy. In oneembodiment, the sacrificial material layers 42 comprise, and/or consistessentially of, silicon nitride. The thickness of each sacrificialmaterial layer 42 can be in a range from 15 nm to 60 nm, although lesserand greater thicknesses can also be employed.

The channel-level insulating layers 32A are a first subset of theinsulating layers 32 on which vertical semiconductor channels aresubsequently formed. The inter-transistor-level insulating layers 32Bare a second subset of the insulating layers 32 which are disposedbetween vertically adjacent pairs of vertical field effect transistors.

In the illustrated example, the alternating stack (32, 42) of theinsulating layers 32 and the sacrificial material layers 42 may beginwith a bottommost sacrificial layer 42, which underlies a bottommostchannel-level insulating layer 32A, and may terminate with a topmostsacrificial layer 42, which overlies a topmost channel-level insulatinglayer 32A. However, an alternative bottommost layer and/or analternative topmost layer may be employed for the alternating stack (32,42). The number of channel-level insulating layers 32A within thealternating stack (32, 42) may be in a range from 2 to 1,024, andtypically from 8 to 256, although a greater number of repetitions mayalso be employed. In one embodiment, the alternating stack (32, 42) mayinclude multiple repetition of a unit layer stack (42, 32A, 42, 43B) toprovide a periodic structure that is repeated along the verticaldirection.

An insulating cap layer 70 may be formed over the alternating stack (32,42). The insulating cap layer 70 includes a dielectric material that isdifferent from the material of the sacrificial material layers 42. Inone embodiment, the insulating cap layer 70 may include a dielectricmaterial that may be employed for the insulating layers 32 as describedabove. The insulating cap layer 70 may have a greater thickness thaneach of the insulating layers 32. The insulating cap layer 70 may bedeposited, for example, by chemical vapor deposition. In one embodiment,the insulating cap layer 70 may be a silicon oxide layer.

Generally, the set of all insulating layers 32 within the alternatingstack (32, 42) comprises a vertically alternating sequence of thechannel-level insulating layers 32A and inter-transistor-levelinsulating layers 32B. In other words, if all the insulating layers 32are selected from the alternating stack (32, 42), the insulating layers32 include a vertically alternating sequence of the channel-levelinsulating layers 32A and inter-transistor-level insulating layers 32B.Each instance of the channel-level insulating layers 32A is formed as ahorizontally-extending layer having a respective top surface and arespective bottom surface, and each instance of theinter-transistor-level insulating layers 32B is formed as ahorizontally-extending layer having a respective top surface and arespective bottom surface. In one embodiment, each sacrificial materiallayer 42 other than a bottommost one of the sacrificial material layers42 and other than a topmost one of the sacrificial material layers 42contacts a respective overlying one of the insulating layers 32 and arespective underlying one of the insulating layers 32.

Referring to FIG. 3, stepped surfaces are formed at a peripheral regionof the alternating stack (32, 42), which is herein referred to as aterrace region. As used herein, “stepped surfaces” refer to a set ofsurfaces that include at least two horizontal surfaces and at least twovertical surfaces such that each horizontal surface is adjoined to afirst vertical surface that extends upward from a first edge of thehorizontal surface, and is adjoined to a second vertical surface thatextends downward from a second edge of the horizontal surface. A steppedcavity is formed within the volume from which portions of thealternating stack (32, 42) are removed through formation of the steppedsurfaces. A “stepped cavity” refers to a cavity having stepped surfaces.

The terrace region is formed in the staircase region 300, which islocated between the memory array region 100 and the peripheral deviceregion 200 containing the at least one semiconductor device for theperipheral circuitry. The stepped cavity may have various steppedsurfaces such that the horizontal cross-sectional shape of the steppedcavity changes in steps as a function of the vertical distance from thetop surface of the substrate 9. In one embodiment, the stepped cavitymay be formed by repetitively performing a set of processing steps. Theset of processing steps may include, for example, an etch process of afirst type that vertically increases the depth of a cavity by one ormore levels, and an etch process of a second type that laterally expandsthe area to be vertically etched in a subsequent etch process of thefirst type. As used herein, a “level” of a structure includingalternating plurality is defined as the relative position of a pair of afirst material layer and a second material layer within the structure.

Each sacrificial material layer 42 other than a topmost sacrificialmaterial layer 42 within the alternating stack (32, 42) laterallyextends farther than any overlying sacrificial material layer 42 withinthe alternating stack (32, 42) in the terrace region. The terrace regionincludes stepped surfaces of the alternating stack (32, 42) thatcontinuously extend from a bottommost layer within the alternating stack(32, 42) to a topmost layer within the alternating stack (32, 42).

Each vertical step of the stepped surfaces may have the height of one ormore pairs of an insulating layer 32 and a sacrificial material layer.In one embodiment, each vertical step may have the height of a singlepair of an insulating layer 32 and a sacrificial material layer 42. Inanother embodiment, multiple “columns” of staircases may be formed alonga first horizontal direction hd1 such that each vertical step has theheight of a plurality of pairs of an insulating layer 32 and asacrificial material layer 42, and the number of columns may be at leastthe number of the plurality of pairs. Each column of staircase may bevertically offset from each other such that each of the sacrificialmaterial layers 42 has a physically exposed top surface in a respectivecolumn of staircases. In the illustrative example, two columns ofstaircases are formed for each block of memory stack structures to besubsequently formed such that one column of staircases providephysically exposed top surfaces for odd-numbered sacrificial materiallayers 42 (as counted from the bottom) and another column of staircasesprovide physically exposed top surfaces for even-numbered sacrificialmaterial layers (as counted from the bottom). Configurations employingthree, four, or more columns of staircases with a respective set ofvertical offsets of the physically exposed surfaces of the sacrificialmaterial layers 42 may also be employed. Each sacrificial material layer42 has a greater lateral extent, at least along one direction, than anyoverlying sacrificial material layers 42 such that each physicallyexposed surface of any sacrificial material layer 42 does not have anoverhang. In one embodiment, the vertical steps within each column ofstaircases may be arranged along the first horizontal direction hd1, andthe columns of staircases may be arranged along a second horizontaldirection hd2 that is perpendicular to the first horizontal directionhd1. In one embodiment, the first horizontal direction hd1 may beperpendicular to the boundary between the memory array region 100 andthe staircase region 300. p A retro-stepped dielectric material portion65 (i.e., an insulating fill material portion) may be formed in thestepped cavity by deposition of a dielectric material therein. Forexample, a dielectric material such as silicon oxide may be deposited inthe stepped cavity. Excess portions of the deposited dielectric materialmay be removed from above the top surface of the insulating cap layer70, for example, by chemical mechanical planarization (CMP). Theremaining portion of the deposited dielectric material filling thestepped cavity constitutes the retro-stepped dielectric material portion65. As used herein, a “retro-stepped” element refers to an element thathas stepped surfaces and a horizontal cross-sectional area thatincreases monotonically as a function of a vertical distance from a topsurface of a substrate on which the element is present. If silicon oxideis employed for the retro-stepped dielectric material portion 65, thesilicon oxide of the retro-stepped dielectric material portion 65 may,or may not, be doped with dopants such as B, P, and/or F.

Referring to FIGS. 4A and 4B, a lithographic material stack (not shown)including at least a photoresist layer may be formed over the insulatingcap layer 70 and the retro-stepped dielectric material portion 65, andmay be lithographically patterned to form openings therein. The openingsinclude a first set of openings formed over the memory array region 100and a second set of openings formed over the staircase region 300. Thepattern in the lithographic material stack may be transferred throughthe insulating cap layer 70 or the retro-stepped dielectric materialportion 65, and through the alternating stack (32, 42) by at least oneanisotropic etch that employs the patterned lithographic material stackas an etch mask. Portions of the alternating stack (32, 42) underlyingthe openings in the patterned lithographic material stack are etched toform memory openings 49 and support openings 19. As used herein, a“memory opening” refers to a structure in which memory elements, such asa memory stack structure, is subsequently formed. As used herein, a“support opening” refers to a structure in which a support structure(such as a support pillar structure) that mechanically supports otherelements is subsequently formed. The memory openings 49 are formedthrough the insulating cap layer 70 and the entirety of the alternatingstack (32, 42) in the memory array region 100. The support openings 19are formed through the retro-stepped dielectric material portion 65 andthe portion of the alternating stack (32, 42) that underlie the steppedsurfaces in the staircase region 300.

The memory openings 49 extend through the entirety of the alternatingstack (32, 42). The support openings 19 extend through a subset oflayers within the alternating stack (32, 42). The chemistry of theanisotropic etch process employed to etch through the materials of thealternating stack (32, 42) may alternate to optimize etching of thefirst and second materials in the alternating stack (32, 42). Theanisotropic etch may be, for example, a series of reactive ion etches.The sidewalls of the memory openings 49 and the support openings 19 maybe substantially vertical, or may be tapered. The patterned lithographicmaterial stack may be subsequently removed, for example, by ashing.

The memory openings 49 and the support openings 19 may extend from thetop surface of the alternating stack (32, 42) to a respective bottomsurface located within the bottom insulating layer 31. Each of thememory openings 49 and the support openings 19 may include a sidewall(or a plurality of sidewalls) that extends substantially perpendicularto the topmost surface of the substrate. A two-dimensional array ofmemory openings 49 may be formed in the memory array region 100. Atwo-dimensional array of support openings 19 may be formed in thestaircase region 300.

Referring to FIG. 5, an isotropic etch process can be performed tolaterally recess physically exposed cylindrical sidewalls of thechannel-level insulating layers 32A selective to the materials of thesacrificial material layers 42 and the inter-transistor-level insulatinglayers 32B. For example, if the sacrificial material layers 42 includesilicon nitride, the inter-transistor-level insulating layers 32Binclude undoped silicate glass, and the channel-level insulating layersinclude a doped silicate glass or organosilicate glass, a wet etchprocess employing a dilute hydrofluoric acid at a dilution of 100:1 orat a higher dilution can be performed to laterally recess the physicallyexposed sidewalls of the channel-level insulating layers 32A. Thelateral recess distance of the sidewalls of the channel-level insulatinglayers 32A can be greater than the lateral recess distance of thesidewalls of the inter-transistor-level insulating layers 32B by afactor of at least three, and/or by a factor of at least 10, such as afactor of 100 or greater.

Optionally, another isotropic etch process may be performed to recessthe sidewalls of the sacrificial material layers 42 so that the recessedsidewalls of the sacrificial material layers 42 are approximatelyvertically coincident with the sidewalls of the inter-transistor-levelinsulating layers 32B. A vertical stack of annular cylindrical cavities47 can be formed around each memory opening 49 in the memory arrayregion 100. Each annular cylindrical cavity 47 can have an outerboundary that coincides with a recessed sidewall of a channel-levelinsulating layer 32A and an inner boundary that coincides with acylindrical vertical plane that includes sidewalls of the sacrificialmaterial layers 42 around a respective memory opening 49. The lateralrecess distance between the sidewalls of the channel-level insulatinglayers 32A and the sidewalls of the sacrificial material layers 42 canbe in a range from 3 nm to 40 nm, such as from 5 nm to 20 nm, althoughlesser and greater recess distances can also be employed.

Referring to FIG. 6, a semiconductor material such as amorphous siliconor polysilicon can be conformally deposited in the annular cylindricalcavities 47 formed by lateral recessing of the channel-level insulatinglayers 32A. The semiconductor material can have a doping of a firstconductivity type, which may be p-type or n-type. The atomicconcentration of dopants of the first conductivity type in thesemiconductor material can be in a range from 1.0×10¹⁴/cm³ to1.0×10¹⁸/cm³, although lesser and greater dopant concentrations can alsobe employed. The thickness of the conformally deposited semiconductormaterial can be selected such that the entire volume of each annularcylindrical cavity 47 is filled with the semiconductor material. Ananisotropic etch that etches the semiconductor material selective to theinsulating materials of the insulating cap layer 70 and the bottominsulating layer 31 can be performed to remove portions of thesemiconductor material that are not masked by the insulating cap layer70. Each remaining tubular portion of the semiconductor material thatfills a respective annular cylindrical cavity 47 constitutes asemiconductor channel 60. Each semiconductor channel 60 can have atubular shape.

Each instance of the semiconductor channel 60 can be formed on asidewall of a respective instance of the channel-level insulating layer32A. In one embodiment, each semiconductor channel 60 can have acylindrical outer sidewall and an inner cylindrical sidewall that islaterally offset inward from the cylindrical outer sidewall by a uniformlateral offset distance, which can be the same as the lateral offsetdistance of the isotropic etch process that forms the recessed sidewallsof the channel-level insulating layers 32A at the processing steps ofFIG. 5. In one embodiment, each semiconductor channel 60 can have anannular planar top surface and an annular planar bottom surface. Theouter peripheries of the annular planar top surface and the annularplanar bottom surface of a semiconductor channel 60 are adjoined to anupper periphery and a lower periphery of a cylindrical outer sidewall ofthe semiconductor channel 60. The inner peripheries of the annularplanar top surface and the annular planar bottom surface of asemiconductor channel 60 are adjoined to an upper periphery and a lowerperiphery of a cylindrical inner sidewall of the semiconductor channel60. The height of each semiconductor channel 60 can be the same as thethickness of the channel-level insulating layer 32A that a cylindricalouter sidewall of the semiconductor channel 60 contacts.

Referring to FIGS. 7A and 7B, a gate dielectric layer 54 is formed oninner sidewalls of each semiconductor channel 60 within a vertical stackof semiconductor channels around each memory opening 49. The gatedielectric layer 54 can be formed directly on physically exposedcylindrical sidewalls of the sacrificial material layers 42 and theinter-transistor-level insulating layers 32B. The gate dielectric layer54 vertically extends as a continuous material layer from a bottommostone of the sacrificial material layers 42 to a topmost one of thesacrificial material layers 42 within each memory opening 49, anddirectly contacts sidewalls of each of the inter-transistor-levelinsulating layers 32B. The gate dielectric layer 54 is laterally spacedfrom each of the channel-level insulating layers 32 by a respectivevertical stack of semiconductor channels 54 located around each memoryopening 49.

The gate dielectric layer 54 includes a ferroelectric material. In oneembodiment, the gate electric layer 54 consists essentially of only theferroelectric material. In another embodiment, the gate dielectric layer54 comprises a multi-film stack of a ferroelectric material film and anon-ferroelectric dielectric material film (e.g., silicon oxide film).As used herein, a “ferroelectric material” refers to a crystallinematerial that exhibits spontaneous electrical polarization in theabsence of an external electric field. The ferroelectric material in thegate dielectric layer 54 may be an insulating ferroelectric material. Inone embodiment, the gate dielectric layer 54 comprises a hafnium oxidelayer including at least one dopant selected from Al, Zr, and Si andhaving a ferroelectric non-centrosymmetric orthorhombic phase.Alternatively, other ferroelectric materials, such as barium titanate,bismuth ferrite, lead titanate, lead zirconate titanate, etc., may beused. The gate dielectric layer 54 may have a thickness in a range from1.5 nm to 15 nm, such as from 3 nm to 10 nm, although lesser and greaterthicknesses can also be employed. In one embodiment, the gate dielectriclayer 54 can be deposited by a conformal deposition process such aschemical vapor deposition (CVD) or atomic layer deposition (ALD).

A bipolar selector material layer 56 is subsequently formed on the gatedielectric layer 54 as a conformal material layer. As used herein, a“bipolar selector material” refers to any material that can function asan on/off switch depending on the magnitude of the applied bias voltagethereacross without switching bias polarity dependence (i.e., thematerial switches under either positive or negative voltage if theabsolute value of the voltage is greater than a threshold value). Theselector material layer can include a non-Ohmic material that provideselectrical connection of electrical isolation depending on the magnitudeof an externally applied voltage bias thereacross. In one embodiment,the selector material layer includes at least one threshold switchmaterial layer. The at least one threshold switch material layerincludes any suitable threshold switch material which exhibitsnon-linear electrical behavior, such as an ovonic threshold switch (OTS)material.

As used herein, an ovonic threshold switch is a device that does notcrystallize in a low resistivity state under a voltage above thethreshold voltage, and reverts back to a high resistivity state when notsubjected to a voltage above a critical holding voltage across the OTSmaterial layer. As used herein, an “ovonic threshold switch material”refers to a material that displays a non-linear resistivity curve underan applied external bias voltage such that the resistivity of thematerial decreases with the magnitude of the applied external biasvoltage. In other words, an ovonic threshold switch material isnon-Ohmic, and becomes more conductive under a higher external biasvoltage than under a lower external bias voltage.

An ovonic threshold switch material (OTS material) can benon-crystalline (for example, amorphous) in a high resistivity state,and can remain non-crystalline (for example, remain amorphous) in a lowresistivity state during application of a voltage above its thresholdvoltage across the OTS material. The OTS material can revert back to thehigh resistivity state when the high voltage above its threshold voltageis lowered below a critical holding voltage. Throughout the resistivitystate changes, the ovonic threshold switch material can remainnon-crystalline (e.g., amorphous).

In one embodiment, the ovonic threshold switch material can comprise achalcogenide material which exhibits hysteresis in both the write andread current polarities. The chalcogenide material may be a GeTecompound or a Ge-Se compound doped with a dopant selected from As, N,and C, such as a Ge—Se—As compound semiconductor material. The ovonicthreshold switch material layer can contain any ovonic threshold switchmaterial. In one embodiment, the ovonic threshold switch material layercan include a compound of at least one Group 14 elements and at leastone Group 16 element. In one embodiment, the ovonic threshold switchmaterial layer can include, and/or can consist essentially of, amaterial selected from a GeSeAs alloy, a GeTeAs, a GeSeTeSe alloy, aGeSe alloy, a SeAs alloy, a GeTe alloy, and a SiTe alloy.

In one embodiment, the material of the selector material layer can beselected such that the resistivity of the selector material thereindecreases at least by two orders of magnitude (i.e., by more than afactor of 100) upon application of an external bias voltage that exceedsa critical bias voltage magnitude (also referred to as thresholdvoltage). In one embodiment, the composition and the thickness of theselector material layer can be selected such that the critical biasvoltage magnitude can be in a range from 1 V to 6 V, although lesser andgreater voltages can also be employed for the critical bias voltagemagnitude. The thickness of the selector material layer can be, forexample, in a range from 1 nm to 50 nm, such as from 5 nm to 25 nm,although lesser and greater thicknesses can also be employed.

A conductive material can be deposited in each remaining unfilled volumeof the memory openings 49 and the support openings. The conductivematerial can include at least one metallic material and/or a heavilydoped semiconductor material. For example, the conductive material caninclude an optional conductive metallic nitride liner (such as a TiNliner, a TaN liner, or a WN liner) and a metal fill material (such astungsten).

Excess portions of the conductive material, the selector material layer56, and the gate dielectric layer 54 can be removed from above ahorizontal plane including the top surface of the insulating cap layer70 by a planarization process, which can employ at least one recess etchprocess and/or a chemical mechanical planarization (CMP) process. Thetop surface of the insulating cap layer 70 can be employed as thestopping layer for the at least one recess etch process and/or the CMPprocess. Each remaining portion of the conductive material in the memoryopenings 49 and the support openings 19 constitutes a word line 62.Portions of each word line 62 located at the level of each semiconductorchannel comprise gate electrodes 62G of respective vertical field effecttransistors 80.

A layer stack including, from one side to another, a gate dielectriclayer 54, a selector material layer 56, and a word line 62 can be formedon a respective vertical stack of semiconductor channels 60 within eachmemory opening 49. Each vertical stack of semiconductor channels 60surrounds a respective one of the memory openings 49, and contacts eachof the channel-level insulating layers 32A. The gate dielectric layer 54comprises a ferroelectric dielectric material, and the selector materiallayer 56 comprises a bipolar switching material providing a voltagemagnitude dependent switching characteristic.

Each gate dielectric layer 54 comprises a tubular gate dielectricportion extending through each layer of the alternating stack (32, 42)and a bottom gate dielectric base portion adjoined to a bottom peripheryof the tubular gate dielectric portion. Each selector material layer 56is laterally surrounded by the tubular gate dielectric portion andcomprises a tubular selector material portion extending through eachlayer of the alternating stack (32, 42) and a bottom selector materialbase portion adjoined to a bottom periphery of the tubular selectormaterial portion. Each word line 62 is laterally surrounded by thetubular selector material portion of a respective selector materiallayer 56, and extends through each layer within the alternating stack(32, 42). Each of the semiconductor channels 60, the gate dielectriclayers 54, the selector material layers 56, and the word lines 62 extendalong a vertical direction.

Each combination of gate dielectric layer 54, the selector materiallayer 56 and the word line 62 that fills a memory opening 49 and anadjoining vertical stack of semiconductor channels 60 togetherconstitute a memory opening fill structure 58. Each combination of allmaterial portions that fills a support opening 19 and an adjoiningvertical stack of semiconductor channels 60 constitutes a support pillarstructure. Two-dimensional arrays of memory opening fill structures 58can be formed in the memory array region 100. Array of support pillarstructures 20 can be formed in the staircase region 300. Thesemiconductor channels 60 are not present on portions of the supportpillar structures 20 that extend through the retro-stepped dielectricmaterial portion 65.

Referring to FIGS. 8A and 8B, a contact level dielectric layer 73 may beformed over the alternating stack {(32A, 32B), 42} of insulating layer(32A, 32B) and sacrificial material layers 42, and over the memoryopening fill structures 58 and the support pillar structures 20. Thecontact level dielectric layer 73 includes a dielectric material that isdifferent from the dielectric material of the sacrificial materiallayers 42. For example, the contact level dielectric layer 73 mayinclude silicon oxide. The contact level dielectric layer 73 may have athickness in a range from 50 nm to 500 nm, although lesser and greaterthicknesses may also be employed.

A photoresist layer (not shown) may be applied over the contact leveldielectric layer 73, and is lithographically patterned to form openingsin areas between clusters of memory opening fill structures 58. Thepattern in the photoresist layer may be transferred through the contactlevel dielectric layer 73, the alternating stack {(32A, 32B), 42} and/orthe retro-stepped dielectric material portion 65 employing ananisotropic etch to form backside trenches 79, which vertically extendfrom the top surface of the contact level dielectric layer 73 down tothe bottom insulating layer 31, and laterally extend through the memoryarray region 100 and the staircase region 300.

In one embodiment, the backside trenches 79 may laterally extend along afirst horizontal direction hd1 and may be laterally spaced apart fromeach other along a second horizontal direction hd2 that is perpendicularto the first horizontal direction hd1. The memory opening fillstructures 58 may be arranged in rows that extend along the firsthorizontal direction hd1. Each backside trench 79 may have a uniformwidth that is invariant along the lengthwise direction (i.e., along thefirst horizontal direction hd1). The photoresist layer may be removed,for example, by ashing.

Referring to FIG. 9, an etchant that selectively etches the sacrificialmaterial layers 42 with respect to the insulating layers (32A, 32B) maybe introduced into the backside trenches 79, for example, employing anetch process. Backside recesses 43 are formed in volumes from which thesacrificial material layers 42 are removed. The removal of thesacrificial material layers 42 may be selective to the insulating layers(32A, 32B), the material of the retro-stepped dielectric materialportion 65, and the material of the gate dielectric layers 54. In oneembodiment, the sacrificial material layers 42 may include siliconnitride, and the materials of the insulating layers (32A, 32B) and theretro-stepped dielectric material portion 65 may be selected fromsilicon oxide and dielectric metal oxides.

The etch process that removes the sacrificial material layers 42selective to the insulating layers (32A, 32B) and the gate dielectriclayers 54 may be a wet etch process employing a wet etch solution, ormay be a gas phase (dry) etch process in which the etchant is introducedin a vapor phase into the backside trenches 79. For example, if thesacrificial material layers 42 include silicon nitride, the etch processmay be a wet etch process in which the first exemplary structure isimmersed within a wet etch tank including phosphoric acid, which etchessilicon nitride selective to silicon oxide, silicon, and various othermaterials employed in the art. The support pillar structure 20, theretro-stepped dielectric material portion 65, and the memory openingfill structures 58 provide structural support while the backsiderecesses 43 are present within volumes previously occupied by thesacrificial material layers 42.

Each backside recess 43 may be a laterally extending cavity having alateral dimension that is greater than the vertical extent of thecavity. In other words, the lateral dimension of each backside recess 43may be greater than the height of the backside recess 43. A plurality ofbackside recesses 43 may be formed in the volumes from which thesacrificial material layers 42 are removed. The memory openings 49 inwhich the memory opening fill structures 58 are formed are hereinreferred to as front side openings or front side cavities in contrastwith the backside recesses 43.

Each of the plurality of backside recesses 43 may extend substantiallyparallel to the top surface of the substrate 9. A backside recess 43 maybe vertically bounded by a top surface of an underlying insulating layer(32A, 32B) and a bottom surface of an overlying insulating layer (32A,32B). In one embodiment, each backside recess 43 may have a uniformheight throughout.

Referring to FIG. 10, a doped semiconductor material can be deposited inthe backside recesses 43. The semiconductor material can have a dopingof a second conductivity type, which is the opposite of the firstconductivity type. For example, if the first conductivity type isp-type, the second conductivity type is n-type, and vice versa. Theatomic concentration of dopants of the second conductivity type in thesemiconductor material can be in a range from 5.0×10¹⁹/cm³ to2.0×10²¹/cm³, although lesser and greater dopant concentrations can alsobe employed. A plurality of doped semiconductor layers (44, 46) can beformed in the backside recesses. The plurality of doped semiconductorlayers (44, 46) include source layers 44 that contact first annularhorizontal surfaces of the semiconductor channels 60 and drain layers 46that contact second annular horizontal surfaces of the semiconductorchannels 60. The source layers 44 function as a source doping region ofa respective vertical field effect transistor, and the drain layers 46function as a drain doping region of a respective vertical field effecttransistor.

Generally, doped semiconductor regions of the source layers and thedrain layers contacting end portions of a semiconductor channel 60 canfunction as a source region or as a drain region of each vertical fieldeffect transistor 80 depending on the voltage bias scheme and thepolarity of the minority charge carriers that flow through thesemiconductor channel. Thus, while the present disclosure is describedemploying an embodiment in which a source layer 44 are formed at abottom end of each semiconductor channel 60 and a drain layer 46 isformed at a top end of each semiconductor channel, it is understood thatthe positions of the source layer 44 and the drain layer 46 contactingany semiconductor channel 60 may be reversed depending on the voltagebias scheme that is employed across the source layer 44 and the drainlayer 46.

In the illustrated example, each annular bottom surface of asemiconductor channel 60 is contacted by a source layer 44, and eachannular top surface of a semiconductor channel 60 is contacted by adrain layer 46. Each source layer 44 contacts annular bottom surfaces ofa two-dimensional array of semiconductor channels 60 having a respectivecylindrical sidewall that contacts a respective one of the channel-levelinsulating layer 32A. Each drain layer 46 contacts annular top surfacesof a two-dimensional array of semiconductor channels 60 having arespective cylindrical sidewall that contacts a respective one of thechannel-level insulating layer 32A. Each portion of a source layer 44that contacts, and/or is proximal to, an end portion of a semiconductorchannel 60 constitutes a source region. Each portion of a drain layer 46that contacts, and/or is proximal to, an end portion of a semiconductorchannel 60 constitutes a drain region. Each sacrificial material layer42 may be replaced with a source layer 44 or a drain layer 46.

A continuous doped semiconductor layer 46L may be formed on thesidewalls of each backside trench 79 and over the contact leveldielectric layer 73. The continuous doped semiconductor layer 46Lincludes a continuously extending portion of the doped semiconductormaterial that is deposited in the backside trenches 79 or above thecontact level dielectric layer 73. A backside cavity 79′ is present inthe portion of each backside trench 79 that is not filled with thecontinuous doped semiconductor layer 46L.

Referring to FIG. 11, the continuous doped semiconductor layer 46L isetched back from the sidewalls of each backside trench 79 and from abovethe contact level dielectric layer 73, for example, by an isotropic wetetch, an anisotropic dry etch, or a combination thereof. Each remainingportion of the doped semiconductor material in the backside recesses 43constitutes a source layer 44 or a drain layer 46. In one embodiment,the doped semiconductor material of the source layers 44 and the drainlayers 46 can be isotropically etched back, for example, employing a wetetch process that etches the doped semiconductor material selective tothe materials of the insulating layers (32A, 32B), the bottom insulatinglayer 31, the insulating cap layer 70, and the contact-level dielectriclayer 73. In an illustrative example, a wet etch process using hottrimethyl-2 hydroxyethyl ammonium hydroxide (“hot TMY”) or tetramethylammonium hydroxide (TMAH) may be performed to laterally recess thesidewalls of the source layers 44 and the drain layers 46 from thebackside trenches 79. Alternatively, the doped semiconductor material ofthe source layers 44 and the drain layers 46 is not isotropically etchedback.

The lateral recess distance of the sidewalls of the source layers 44 andthe drain layers 46 can be controlled such that surfaces of thesemiconductor channels 60 are not physically exposed, and eachsemiconductor channel 60 is spaced from the volumes of the recessesaround the backside trenches 79 at least by a respective source region(a region of a source layer 44) and by a respective drain region (aregion of a drain layer 46). In one embodiment, the lateral recessdistance of the sidewalls of the source layers 44 and the drain layers46 around each backside trench 79 can be in a range from 20 nm to 100nm, although lesser and greater lateral recess distances can also beemployed.

Referring to FIG. 12, at least one metallic material can be deposited inthe lateral recesses around the backside trenches 79, at peripheralregions of the backside trenches 79, and over the contact-leveldielectric layer 73. The at least one metallic material can include ametallic barrier liner material and a metallic fill material. Themetallic barrier liner material can include a conductive metallicnitride material such as TiN, TaN, WN, or a stack thereof. In oneembodiment, the metallic barrier liner material may be deposited by aconformal deposition process such as chemical vapor deposition (CVD) oratomic layer deposition (ALD). The thickness of the metallic barrierliner material may be in a range from 1 nm to 5 nm, such as from 2 nm to3 nm, although lesser and greater thicknesses may also be employed. Inone embodiment, the metallic barrier liner material may consistessentially of a conductive metal nitride such as TiN. The metallic fillmaterial may be deposited by a conformal deposition method, which maybe, for example, chemical vapor deposition (CVD), atomic layerdeposition (ALD), electroless plating, electroplating, or a combinationthereof. In one embodiment, the metallic fill material may consistessentially of at least one elemental metal. The at least one elementalmetal of the metallic fill material may be selected, for example, fromtungsten, cobalt, ruthenium, titanium, and tantalum. In one embodiment,the metallic fill material may consist essentially of a single elementalmetal.

A plurality of metallic material layers (64, 66) can be formed in theplurality of lateral recesses, and a continuous metallic material layer66L may be formed on the sidewalls of each backside trench 79 and overthe contact level dielectric layer 73. Each metallic material layer (64,66) includes a portion of the metallic barrier layer (which includes themetallic barrier liner material) and a portion of a metallic fillmaterial layer (which includes the metallic fill material). Thecontinuous metallic material layer 66L includes a continuous portion ofthe metallic barrier layer and a continuous portion of the metallic fillmaterial layer that are located in the backside trenches 79 or above thecontact level dielectric layer 73.

Each metallic material layer 64 that contacts a source layer 44constitutes a source contact layer 64, and each metallic material layer66 that contacts a drain layer 46 constitutes a drain contact layer 66.Each of the source contact layers 64 and the drain contact layers 66 canhave a uniform width (which is the lateral recess distance at theprocessing steps of FIG. 11), and can laterally extend along the firsthorizontal direction hd1. Formation of the metallic material layers (64,66) is optional. In case the doped semiconductor material of the sourcelayers 44 and the drain layers 46 is not isotropically etched back atthe step shown in FIG. 11, then the metallic material layers (64, 66)are omitted. In this case, the source layer 44 fills the entirety of abackside recess 43 at a source level and the drain layer 46 fills theentirety of a backside recess 43 at a drain level.

Referring to FIGS. 13A and 13B, the deposited metallic material of thecontinuous metallic material layer 66L is etched back from the sidewallsof each backside trench 79 and from above the contact level dielectriclayer 73, for example, by an isotropic wet etch, an anisotropic dryetch, or a combination thereof. Each remaining portion of the at leastone deposited metallic material in the backside recesses 43 constitutesa source contact layer 64 or a drain contact layer 66. Thus, eachsacrificial material layers 42 is replaced with a respective replacementmaterial layer {(44, 46), (64, 66)}. Each replacement material layerincludes a combination of a source layer 44 and a source contact layer64 or a combination of a drain layer 46 and a drain contact layer 66.The source contact layers 64 and the drain contact layers 66 can includea respective metallic material layer.

Generally, a source region 44S comprising a portion of a source layer 44is formed on one end of each semiconductor channel 60, and a drainregion 46D comprising a portion of a drain layer 46) is formed onanother end of each semiconductor channel 60. At least one contiguouscombination, which may be a plurality of contiguous combinations, of asemiconductor channel 60, a source region 44S, and a drain region 46Dcan be formed on each gate dielectric layer 54. The total number ofcontiguous combinations of a semiconductor channel 60, a source region44S, and a drain region 46D contacting each gate dielectric layer 54 ineach memory opening 49 can be the same as the total number ofchannel-level insulating layers 32A within the alternating stack ofinsulating layers (32A, 32B) and electrically conductive layers {(44,64), (46, 66)}.

Each of the replacement material layers {(44, 46), (64, 66)} includes arespective source layer 44 containing source regions 44S or a respectivedrain layer 46 containing drain regions 46D. The source layers 44 andthe drain layers 46 are formed on each of the gate dielectric layers 54located in the memory openings 49. The metallic material layers (64, 66)are formed on a sidewall of a respective one of the source layers 44 andthe drain layers 46, and are laterally spaced from the gate dielectriclayers 54. Each of the metallic material layers (64, 66) contacts asidewall of a respective one of the source layers 44 and the drainlayers 46, and does not contact the gate dielectric layers 54. In oneembodiment, each metallic material layer (64, 66) other than a topmostone of the metallic material layers (64, 66) and other than a bottommostone of the metallic material layers (64, 66) contacts a respective oneof the channel-level insulating layers 32A and a respective one of theinter-transistor-level insulating layers 32B.

Each combination of a source layer 44 and a source contact layer 64constitutes a source-side electrically conductive layer (44, 64). Eachcombination of a drain layer 46 and drain contact layer 66 constitutes adrain-side electrically conductive layer (46, 66). The source-sideelectrically conductive layers (44, 64) and the drain-side electricallyconductive layers (46, 66) are collectively referred to as electricallyconductive layers {(44, 46), (64, 66)}.

A plurality of contiguous combinations of a respective semiconductorchannel 60, a respective source region 44S, and a respective drainregion 46D can be arranged along the vertical direction on a sidewall ofa respective gate dielectric layer 54 around each memory opening 49.Multiple inter-transistor-level insulating layer 32B can be providedwithin the alternating stack of the insulating layers (32A, 32B) and theelectrically conductive layers {(44, 64), (46, 66)} over the substrate 9such that vertically neighboring pairs of combinations of asemiconductor channel 60, a source region 44S, and a drain region 46Dare vertically spaced apart by a respective inter-transistor-levelinsulating layer 32B.

Referring to FIG. 14, a dielectric material such as silicon oxide can bedeposited in the backside trenches to form dielectric wall structures76. Each dielectric wall structure 76 can contact sidewalls of eachinsulating layer (32A, 32B) within an alternating stack of theinsulating layers (32A, 32B) and the electrically conductive layers{(44, 64), (46, 66)}, and can contact each metallic material layer (64,66) within the alternating stack. Each dielectric wall structure 76 islaterally spaced from the source layers 44 and the drain layers 46. Thetop surface of each dielectric wall structure 76 may be coplanar withthe top surface of the contact-level dielectric layer 73. Alternatively,a horizontal portion of the dielectric material that overlie the topsurface of the contact-level dielectric layer 73 as provided at theprocessing steps of FIGS. 13A and 13B may be incorporated into thecontact-level dielectric layer 73.

Each ferroelectric material portion of the gate dielectric layers 54contacting a respective one of the semiconductor channels constitutes amemory element that can store the direction of polarization as a databit. The polarization of each ferroelectric material portion determinesthe threshold voltage of a vertical field effect transistor 80 includingthe respective ferroelectric material portion and an adjoinedsemiconductor channel 60. If the polarization direction in aferroelectric material portion is such that the ferroelectric materialportion attracts minority charge carriers in the adjoined semiconductorchannel 60, the field effect transistor 80 has a low threshold voltage.If the polarization direction in a ferroelectric material portion issuch that the ferroelectric material portion repels minority chargecarriers in the adjoined semiconductor channel 60, the field effecttransistor 80 has a high threshold voltage.

Referring to FIGS. 15A and 15B, contact via structures (88, 86, 8P) maybe formed through the contact level dielectric layer 73, and optionallythrough the retro-stepped dielectric material portion 65. For example,gate contact via structures 88 may be formed through the contact leveldielectric layer 73 on a top surface of a respective word line 62 in thememory array region 100. Source/drain contact via structures 86 may beformed on a respective one of the source layers 44 and the drain layers46 through the contact level dielectric layer 73, and through theretro-stepped dielectric material portion 65 in the staircase region300. Peripheral device contact via structures 8P may be formed throughthe retro-stepped dielectric material portion 65 directly on respectivenodes of the peripheral devices 700 in the peripheral region 200. In onealternative embodiment, the peripheral devices 700 are formed on aseparate substrate and then bonded to the memory devices. In that case,the peripheral device contact via structures 8P may be omitted. Inanother alternative embodiment, the peripheral devices 700 may be formedbetween the bottom insulating layer 31 and the substrate 9, and theperipheral device contact via structures 8P can connect various nodes ofthe peripheral devices 700 to the various nodes of a three-dimensionalarray of ferroelectric memory elements.

Referring to FIG. 16, a second exemplary structure according to a secondembodiment of the present disclosure is illustrated, which can bederived from the first exemplary structure of FIG. 1 by forming a bottominsulating layer 31 and a periodic sequence of material layers includingmultiple repetitions of a unit layer stack. The unit layer stack caninclude a first instance of a sacrificial material layer 42, a firstinstance of a doped semiconductor layer that can be a source layer 44, achannel-level insulating layer 32A, a second instance of the dopedsemiconductor layer that can be a drain layer 46, a second instance of asacrificial material layer 42and an inter-transistor-level insulatinglayer 32B. An inter-transistor-level insulating layer 32B of the topmostrepetition of the unit layer stack can be replaced with an insulatingcap layer 70.

Semiconductor channels to be subsequently formed have a doping of afirst conductivity type. The doped semiconductor layers (44, 46) have adoping of the second conductivity type, which is the opposite of thefirst conductivity type. The atomic concentration of the dopants of thesecond conductivity type in the doped semiconductor layers (44, 46) canbe in a range from 5.0×10¹⁹/cm³ to 2.0×10²¹/cm³, although lesser andgreater dopant concentrations can also be employed. The thickness ofeach doped semiconductor layer (44, 46) can be in a range from 10 nm to50 nm, although lesser and greater thicknesses can also be employed.

Each doped semiconductor layer (44, 46) is formed between, and contacts,a vertically neighboring pair of one of the channel-level insulatinglayers 32A and one of the sacrificial material layers 42. Each of thedoped semiconductor layers (44, 46) can comprise a source layer 44 or adrain layer 44. Each source layer 44 includes a two-dimensional array ofsource regions for a two-dimensional array of semiconductor channels tobe subsequently formed, and each drain layer 46 includes atwo-dimensional array of drain regions for a two-dimensional array ofsemiconductor channels to be subsequently formed.

Referring to FIG. 17, the processing steps of FIG. 3 can be performedwith a modification such that the anisotropic etch processes that formsthe vertical steps extending through a combination of a sacrificialmaterial layer 42 and an insulating layer 32 are modified to providevertical steps extending through a contiguous combination of asacrificial material layer 42, a drain layer 46, and a transistor-levelinsulating layer 32A, or through a contiguous combination of a sourcelayer 44, a sacrificial material layer 42, and an inter-transistor-levelinsulating layer 32B. In this case, the terminal step of eachanisotropic etch process that forms the vertical steps of the variouslayers in the layer stack including multiple repetitions of the unitlayer stack (42, 44, 32A, 46, 42, 32B) can be selective to the materialof the sacrificial material layers 42 or selective to the material ofthe source layers 44. A retro-stepped dielectric material portion 65 canbe formed in the stepped cavity to form a retro-stepped dielectricmaterial portion 65.

Referring to FIG. 18, the processing steps of FIGS. 4A and 4B can beperformed with a modification to the etch chemistry of the anisotropicetch process to etch through unmasked portions of the insulating caplayer 70 and the layer stack including multiple repetitions of the unitlayer stack (42, 44, 32A, 46, 42, 32B). The bottom insulating layer 31can be employed as an etch stop structure. Memory openings 49 are formedin the memory array region 100, and support openings 19 are formed inthe staircase region 300.

Referring to FIG. 19, an isotropic etch process can be performed tolaterally recess physically exposed cylindrical sidewalls of thechannel-level insulating layers 32A selective to the materials of thesacrificial material layers 42, the doped semiconductor layers (44, 46),and the inter-transistor-level insulating layers 32B, as described abovewith respect to FIG. 4. The semiconductor channels 60 are then formed inthe recesses as described above with respect to FIG. 5.

One end of each semiconductor channel 60 contacts a horizontal surfaceof a source layer 44, and another end of each semiconductor channel 60contacts a horizontal surface of a drain layer 46. Each portion of thesource layers 44 that contacts, and/or is proximal to, a semiconductorchannel 60 constitutes a source region 44S, and each portion of thedrain layers 46 that contacts, and/or is proximal to, a semiconductorchannel 60 constitutes a drain region 46D. Inner cylindrical sidewallsof the semiconductor channels 60 can be vertically coincident withoverlying or underlying sidewalls of the insulating layers 32 and thedoped semiconductor layers (44, 46).

Referring to FIG. 20, the processing steps of FIGS. 7A and 7B can beperformed to form a layer stack including a gate dielectric layer 54, aselector material layer 56, and a word line 62 in each of the memoryopenings 49 and the support openings 19. The semiconductor channels 60are not present on portions of the support pillar structures 20 thatextend through the retro-stepped dielectric material portion 65.

Referring to FIG. 21, a contact level dielectric layer 73 may be formedover the insulating cap layer 70, the memory opening fill structures 58,and the support pillar structures 20. A photoresist layer (not shown)may be applied over the contact level dielectric layer 73, and islithographically patterned to form openings in areas between clusters ofmemory opening fill structures 58. The pattern in the photoresist layermay be transferred through the contact level dielectric layer 73, themultiple repetitions of the unit layer stack (42, 44, 32A, 46, 42, 32B)and/or the retro-stepped dielectric material portion 65 employing ananisotropic etch to form backside trenches 79, which vertically extendfrom the top surface of the contact level dielectric layer 73 down tothe bottom insulating layer 31, and laterally extend through the memoryarray region 100 and the staircase region 300.

In one embodiment, the backside trenches 79 may laterally extend along afirst horizontal direction hd1 and may be laterally spaced apart fromeach other along a second horizontal direction hd2 that is perpendicularto the first horizontal direction hd1. The memory opening fillstructures 58 may be arranged in rows that extend along the firsthorizontal direction hd1. Each backside trench 79 may have a uniformwidth that is invariant along the lengthwise direction (i.e., along thefirst horizontal direction hd1). The photoresist layer may be removed,for example, by ashing.

Referring to FIG. 22, an etchant that selectively etches the sacrificialmaterial layers 42 with respect to the insulating layers (32A, 32B) andthe doped semiconductor layers (44, 46) may be introduced into thebackside trenches 79, for example, employing an etch process. Backsiderecesses 63 are formed in volumes from which the sacrificial materiallayers 42 are removed. The removal of the sacrificial material layers 42may be selective to the material of the insulating layers (32A, 32B),the material of the doped semiconductor layers (44, 46), the material ofthe retro-stepped dielectric material portion 65, and the material ofthe gate dielectric layers 54. In one embodiment, the sacrificialmaterial layers 42 may include silicon nitride, and the materials of theinsulating layers (32A, 32B) and the retro-stepped dielectric materialportion 65 may be selected from silicon oxide and dielectric metaloxides.

The etch process that removes the sacrificial material layers 42selective to the insulating layers (32A, 32B), the doped semiconductorlayers (44, 46), and the gate dielectric layers 54 may be a wet etchprocess employing a wet etch solution, or may be a gas phase (dry) etchprocess in which the etchant is introduced in a vapor phase into thebackside trenches 79. For example, if the sacrificial material layers 42include silicon nitride, the etch process may be a wet etch process inwhich the first exemplary structure is immersed within a wet etch tankincluding phosphoric acid, which etches silicon nitride selective tosilicon oxide, silicon, and various other materials employed in the art.The support pillar structure 20, the retro-stepped dielectric materialportion 65, and the memory opening fill structures 58 provide structuralsupport while the backside recesses 63 are present within volumespreviously occupied by the sacrificial material layers 42.

Each backside recess 63 may be a laterally extending cavity having alateral dimension that is greater than the vertical extent of thecavity. A plurality of backside recesses 63 may be formed in the volumesfrom which the sacrificial material layers 42 are removed. The memoryopenings 49 in which the memory opening fill structures 58 are formedare herein referred to as front side openings or front side cavities incontrast with the backside recesses 63.

Each of the plurality of backside recesses 63 may extend substantiallyparallel to the top surface of the substrate 9. A backside recess 63 maybe vertically bounded by a top surface of an underlying insulating layer(such as an inter-transistor-level insulating layer 32B) and a bottomsurface of an overlying doped semiconductor layer (such as a sourcelayer 44), or may be vertically bounded by a top surface of anunderlying doped semiconductor layer (such as a drain layer 46) and abottom surface of an overlying insulating layer (such as aninter-transistor-level insulating layer 32B). In one embodiment, eachbackside recess 63 may have a uniform height throughout.

Referring to FIG. 23, at least one metallic material can be deposited inthe backside recesses 63, at peripheral regions of the backside trenches79, and over the contact-level dielectric layer 73. The at least onemetallic material can include a metallic barrier liner material and ametallic fill material. The metallic barrier liner material can includea conductive metallic nitride material such as TiN, TaN, WN, or a stackthereof. In one embodiment, the metallic barrier liner material may bedeposited by a conformal deposition process such as chemical vapordeposition (CVD) or atomic layer deposition (ALD). The thickness of themetallic barrier liner material may be in a range from 1 nm to 5 nm,such as from 2 nm to 3 nm, although lesser and greater thicknesses mayalso be employed. In one embodiment, the metallic barrier liner materialmay consist essentially of a conductive metal nitride such as TiN. Themetallic fill material may be deposited by a conformal depositionmethod, which may be, for example, chemical vapor deposition (CVD),atomic layer deposition (ALD), electroless plating, electroplating, or acombination thereof. In one embodiment, the metallic fill material mayconsist essentially of at least one elemental metal. The at least oneelemental metal of the metallic fill material may be selected, forexample, from tungsten, cobalt, ruthenium, titanium, and tantalum. Inone embodiment, the metallic fill material may consist essentially of asingle elemental metal.

A plurality of metallic material layers (64, 66) can be formed in theplurality of backside recesses 63, and a continuous metallic materiallayer may be formed on the sidewalls of each backside trench 79 and overthe contact level dielectric layer 73. Each metallic material layer (64,66) includes a portion of the metallic barrier layer (which includes themetallic barrier liner material) and a portion of a metallic fillmaterial layer (which includes the metallic fill material). Thecontinuous metallic material layer includes a continuous portion of themetallic barrier layer and a continuous portion of the metallic fillmaterial layer that are located in the backside trenches 79 or above thecontact level dielectric layer 73.

Each metallic material layer 64 that contacts a source layer 44constitutes a source contact layer 64, and each metallic material layer66 that contacts a drain layer 46 constitutes a drain contact layer 66.The area of each source contact layer 64 can be the same as the area ofan overlying source layer 44, and the area of each drain contact layer66 can be the same as the area of an underlying drain layer 46.

The deposited metallic material of the continuous metallic materiallayers are etched back from the sidewalls of each backside trench 79 andfrom above the contact level dielectric layer 73, for example, by anisotropic wet etch, an anisotropic dry etch, or a combination thereof.Each remaining portion of the at least one deposited metallic materialin the backside recesses 63 constitutes a source contact layer 64 or adrain contact layer 66. Thus, each sacrificial material layers 42 isreplaced with a respective replacement material layer that includes,and/or consists of, one of the source contact layers 64 and the draincontact layers 66. The source contact layers 64 and the drain contactlayers 66 can include a respective metallic material layer. Eachcontiguous combination of a source layer 44 and a source contact layer64 constitutes a source-side electrically conductive layer (44, 64), andeach contiguous combination of a drain layer 46 and a drain contactlayer 66 constitutes a drain-side electrically conductive layer (46,66). The source-side electrically conductive layers (44, 64) and thedrain-side electrically conductive layers (46, 66) are collectivelyreferred to as electrically conductive layers {(44, 64), (46, 66)}.

Generally, a source region 44S (comprising a portion of a source layer44) is formed on one end of each semiconductor channel 60, and a drainregion 46D (comprising a portion of a drain layer 46) is formed onanother end of each semiconductor channel 60. At least one contiguouscombination, which may be a plurality of contiguous combinations, of asemiconductor channel 60, a source region 44S, and a drain region 46Dcan be formed on each gate dielectric layer 54. An alternating stack ofinsulating layers (32A, 32B) and electrically conductive layers {(44,64), (46, 66)} can be formed over the substrate 9. The total number ofcontiguous combinations of a semiconductor channel 60, a source region44S, and a drain region 46D contacting each gate dielectric layer 54 canbe the same as the total number of channel-level insulating layers 32Awithin the alternating stack [(32A, 32B), {(44, 64), (46, 66)}].

The source layers 44 and the drain layers 46 are formed on each of thegate dielectric layers 54 located in the memory openings 49, andcontacts horizontal surfaces of the semiconductor channels 60. Themetallic material layers (64, 66) are formed directly on each of thegate dielectric layers 54, and are vertically spaced from thesemiconductor channels 60 by a respective one of the source layers 44and the drain layer 46. Each of the metallic material layers (64, 66)contacts a sidewall of each of the gate dielectric layers 54 that arelaterally enclosed by the metallic material layers (64, 66).

A plurality of contiguous combinations of a respective semiconductorchannel 60, a respective source region 44S, and a respective drainregion 46D can be arranged along the vertical direction on a sidewall ofa respective gate dielectric layer 54 around each memory opening 49.Multiple inter-transistor-level insulating layer 32B can be providedwithin the alternating stack of the insulating layers (32A, 32B) and theelectrically conductive layers {(44, 64), (46, 66)} over the substrate 9such that vertically neighboring pairs of combinations of asemiconductor channel 60, a source region 44S, and a drain region 46Dare vertically spaced apart by a respective inter-transistor-levelinsulating layer 32B.

Referring to FIG. 24, the processing steps of FIGS. 14, 15A, and 15B canbe performed to form dielectric wall structures 76 and various contactvia structures (88, 86, 8P). For example, gate contact via structures 88may be formed through the contact level dielectric layer 73 on a topsurface of a respective word line 62 in the memory array region 100.Source/drain contact via structures 86 may be formed on a respective oneof the source layers 44 and the drain layers 46 through the contactlevel dielectric layer 73, and through the retro-stepped dielectricmaterial portion 65 in the staircase region 300. The metallic materiallayers (64, 66) can be employed as etch stop structures during formationof via cavities for forming the source/drain contact via structure 86.In this case, each of the source/drain contact via structures 86 cancontact a respective one of the source contact layers 44 and the draincontact layers 46. Peripheral device contact via structures 8P may beformed through the retro-stepped dielectric material portion 65 directlyon respective nodes of the peripheral devices 700 in the peripheralregion 200. In one alternative embodiment, the peripheral devices 700are formed on a separate substrate and then bonded to the memorydevices. In that case, the peripheral device contact via structures 8Pmay be omitted. In another alternative embodiment, the peripheraldevices 700 may be formed between the bottom insulating layer 31 and thesubstrate 9, and the peripheral device contact via structures 8P canconnect various nodes of the peripheral devices 700 to the various nodesof a three-dimensional array of ferroelectric memory elements.

Referring to FIG. 25, a schematic circuit diagram of thethree-dimensional array of memory elements according to embodiments ofthe present disclosure is illustrated. Each field effect transistor(i.e., ferroelectric field effect transistor, FeFET) 80A includes asemiconductor channel 60, a source region 44S contacting one end of thesemiconductor channel 60, a drain region 46D contacting the opposite endof the semiconductor channel 60, a gate electrode 62G located adjacentto each semiconductor channel 60, a ferroelectric gate dielectric layer54 located between the semiconductor channel 60 and the gate electrode62G, and a bidirectional selector material layer 56 located between thegate electrode 62G and the ferroelectric gate dielectric layer 54. Thegate electrode 62G may be part of a vertical word line 62 (WL1, WL2).

The source region 44S may comprise a part of a source layer 44, and thedrain region 46D may comprise a portion of a drain layer 46. Each sourcelayer 44 may comprise at least a portion of a source line (44, 64) (SL1,SL2) and each drain layer 46 may comprise at least a portion of a bitline (46, 66) (BL1, BL2).

Each word line 62 (WL1, WL2) controls the gate electrodes 62G of arespective vertical stack of vertical field effect transistors 80, suchas a stack of a first vertical field effect transistor 80A and a secondvertical field effect transistor 80B, etc., or a stack of a thirdvertical field effect transistor 80C and a fourth field effecttransistor 80D, etc., as illustrated in FIG. 25.

In a FeFET without the bidirectional selector material layer 56, theprogram or erase disturb voltage could be one half of the program anderase voltage. By adding the bidirectional selector material layer 56between the gate electrode 62G and the semiconductor channel 60 of aFeFET 80 of the embodiments of the present disclosure, there is noprogram or erase disturb on the FeFET 80. In one embodiment, thebidirectional selector material layer 56 may have a switching thresholdvoltage that is greater than one half of the program and erase voltagethat is needed to program and erase the FeFET 80. If the voltagedifference between word line and bit line voltages is greater than theswitching threshold voltage of the bidirectional selector material layer56, then the semiconductor channel can be activated by the word linevoltage. If the voltage difference between word line and bit linevoltages is less than the switching threshold voltage of thebidirectional selector material layer 56, then the semiconductor channel60 is inactivated (i.e., the gate electrode to semiconductor channelbias is about 0V).

Referring to all drawings and according to various embodiments of thepresent disclosure, a ferroelectric field effect transistor (FeFET) 80(e.g., 80A) includes a semiconductor channel 60, a source region 44Scontacting one end of the semiconductor channel, a drain region 46Dcontacting a second end of the semiconductor channel, a gate electrode62G, a ferroelectric gate dielectric layer 54 located between thesemiconductor channel and the gate electrode, and a bidirectionalselector material layer 56 located between the gate electrode and theferroelectric gate dielectric layer.

In one embodiment, the FeFET also includes a substrate 9 having ahorizontal surface 7, wherein the semiconductor channel, theferroelectric gate dielectric layer, the bidirectional selector materiallayer, and the gate electrode extend along a vertical directionperpendicular to the horizontal surface 7. The gate electrode 62Gcomprises a portion of a vertical word line 62 which extendsperpendicular to the horizontal surface.

In one embodiment, the source region 44S comprises a portion of a sourcelayer 44, the drain region 46D comprises portion of a drain layer 46,the source layer 44 comprises at least a portion of a source line (44,64), and the drain layer 46 comprises at least a portion of a bit line(46, 66). In one embodiment, a channel-level insulating layer 32A islocated between the source region 44S and the drain region 46D.

In one embodiment a three dimensional FeFET array includes the FeFET80A, an additional FeFET 80B located below the FeFET 80A, aninter-transistor-level insulating layer 32B located between the sourceline (44, 64) of the FeFET 80A and a bit line (46,66) of the additionalFeFET 80B.

In one embodiment, the FeFET array comprises an alternating stack ofinsulating layers (32A, 32B) and electrically conductive layers {(44,64), (46, 66)} that alternate along the vertical direction; a set of allinsulating layers (32A, 32B) within the alternating stack comprises avertically alternating sequence of the channel-level insulating layers32A and inter-transistor-level insulating layers 32B; and a set of allelectrically conductive layers {(44, 64), (46, 66)} within thealternating stack comprises a vertically alternating sequence of sourcelayers 44 and drain layers 46.

In one embodiment, each of the metallic material layers (64, 66)contacts a sidewall of a respective one of the source layers 44 and thedrain layers 46, and does not contact the ferroelectric gate dielectriclayer 54. In one embodiment, each of the metallic material layers (64,66) contacts a horizontal surface of a respective one of the sourcelayers 44 and the drain layers 46, and contacts the ferroelectric gatedielectric layer 54.

In one embodiment, a plurality of memory openings 49 can verticallyextend through the alternating stack of the insulating layers (32A, 32B)and the electrically conductive layers {(44, 64), (46, 66)}. Theferroelectric gate dielectric layer 54, the bidirectional selectormaterial layer 56, and the vertical word line 62 are located within amemory opening 49 of the plurality of memory openings 49.

In one embodiment, the ferroelectric gate dielectric layer 54 comprisesa tubular gate dielectric portion extending through each of theelectrically conductive layers {(44, 64), (46, 66)} and a bottom gatedielectric base portion adjoined to a bottom periphery of the tubulargate dielectric portion; the bidirectional selector material layer 56 islaterally surrounded by the tubular gate dielectric portion andcomprises a tubular selector material portion extending through each ofthe electrically conductive layers {(44, 64), (46, 66)} and a bottomselector material base portion adjoined to a bottom periphery of thetubular selector material portion; and the vertical word line 62 islaterally surrounded by the tubular selector material portion andextends through each of the electrically conductive layers {(44, 64),(46, 66)}.

In one embodiment, the bidirectional selector material layer 54comprises an ovonic threshold switch material.

Although the foregoing refers to particular preferred embodiments, itwill be understood that the disclosure is not so limited. It will occurto those of ordinary skill in the art that various modifications may bemade to the disclosed embodiments and that such modifications areintended to be within the scope of the disclosure. Compatibility ispresumed among all embodiments that are not alternatives of one another.The word “comprise” or “include” contemplates all embodiments in whichthe word “consist essentially of” or the word “consists of”replaces theword “comprise” or “include,” unless explicitly stated otherwise. Wherean embodiment employing a particular structure and/or configuration isillustrated in the present disclosure, it is understood that the presentdisclosure may be practiced with any other compatible structures and/orconfigurations that are functionally equivalent provided that suchsubstitutions are not explicitly forbidden or otherwise known to beimpossible to one of ordinary skill in the art. All of the publications,patent applications and patents cited herein are incorporated herein byreference in their entirety.

1. A ferroelectric field effect transistor (FeFET) comprising: asemiconductor channel; a source region contacting one end of thesemiconductor channel; a drain region contacting a second end of thesemiconductor channel; a gate electrode; a ferroelectric gate dielectriclayer located between the semiconductor channel and the gate electrode;and a bidirectional selector material layer located between the gateelectrode and the ferroelectric gate dielectric layer.
 2. The FeFET ofclaim 1, further comprising a substrate having a horizontal surface,wherein: the semiconductor channel, the ferroelectric gate dielectriclayer, the bidirectional selector material layer, and the gate electrodeextend along a vertical direction perpendicular to the horizontalsurface; and the gate electrode comprises a portion of a vertical wordline which extends perpendicular to the horizontal surface.
 3. The FeFETof claim 2, wherein: the source region comprises a portion of a sourcelayer; the drain region comprises portion of a drain layer; the sourcelayer comprises at least a portion of a source line; and the drain layercomprises at least a portion of a bit line.
 4. The FeFET of claim 3,further comprising a channel-level insulating layer located between thesource region and the drain region.
 5. A three dimensional FeFET array,comprising: the FeFET of claim 4; an additional FeFET located below theFeFET; and an inter-transistor-level insulating layer located betweenthe source line of the FeFET and a bit line of the additional FeFET. 6.The three dimensional FeFET array of claim 5, wherein: the arraycomprises alternating stack of insulating layers and electricallyconductive layers that alternate along the vertical direction; a set ofall insulating layers within the alternating stack comprises avertically alternating sequence of the channel-level insulating layersand inter-transistor-level insulating layers; and a set of allelectrically conductive layers within the alternating stack comprises avertically alternating sequence of the source layers and the drainlayers.
 7. The three dimensional FeFET array of claim 6, furthercomprising metallic material layers contacting a respective one of thesource layers and the drain layers and located between verticallyneighboring pairs of the insulating layers.
 8. The three dimensionalFeFET array of claim 7, wherein each of the metallic material layerscontacts a sidewall of a respective one of the source layers and thedrain layers, and does not contact the ferroelectric gate dielectriclayer.
 9. The three dimensional FeFET array of claim 7, wherein each ofthe metallic material layers contacts a horizontal surface of arespective one of the source layers and the drain layers, and contactsthe ferroelectric gate dielectric layer.
 10. The three dimensional FeFETarray of claim 6, further comprising a plurality of memory openingsvertically extending through the alternating stack of the insulatinglayers and the electrically conductive layers, wherein the ferroelectricgate dielectric layer, the bidirectional selector material layer, andthe vertical word line are located within a memory opening of theplurality of memory openings.
 11. The three dimensional FeFET array ofclaim 10, wherein: the ferroelectric gate dielectric layer comprises atubular gate dielectric portion extending through each of theelectrically conductive layers and a bottom gate dielectric base portionadjoined to a bottom periphery of the tubular gate dielectric portion;the bidirectional selector material layer is laterally surrounded by thetubular gate dielectric portion and comprises a tubular selectormaterial portion extending through each of the electrically conductivelayers and a bottom selector material base portion adjoined to a bottomperiphery of the tubular selector material portion; and the verticalword line is laterally surrounded by the tubular selector materialportion and extends through each of the electrically conductive layers.12. The FeFET of claim 1, wherein the bidirectional selector materiallayer comprises an ovonic threshold switch material.
 13. A method offorming a semiconductor device, comprising: forming channel-levelinsulating layers over a substrate; forming a vertical stack ofsemiconductor channels at levels of the channel-level insulating layers;forming a stack including, from one side to another, a ferroelectricgate dielectric layer, a bidirectional selector material layer, and agate electrode directly on the vertical stack of semiconductor channels;and providing a source region on one end of each of the semiconductorchannels and a drain region on another end of each of the semiconductorchannels, wherein each of the semiconductor channels, the sourceregions, and the drain region contacts the ferroelectric gate dielectriclayer, and each of the channel-level insulating layers contacts arespective one of the source regions and a respective one of the drainregion.
 14. The method of claim 13, wherein: each of the ferroelectricgate dielectric layer, the bidirectional selector material layer, andthe gate electrode is formed over the substrate, and extends along avertical direction that is perpendicular to a top surface of thesubstrate; each of the channel-level insulating layers is formed as ahorizontally-extending layer having a top surface and a bottom surface;and each of the semiconductor channels is formed as a tubular structureon a sidewall of a respective one of the channel-level insulatinglayers.
 15. The method of claim 14, further comprising forminginter-transistor-level insulating layers over the substrate such thatthe inter-transistor-level insulating layers are interfaced with thechannel-level insulating layers, wherein: each of the semiconductorcontacts a sidewall of the ferroelectric gate dielectric layer; andvertically neighboring pairs of combinations of a respectivesemiconductor channel, a respective source region, and a respectivedrain region are vertically spaced apart by a respective one of theinter-transistor-level insulating layers.
 16. The method of claim 15,further comprising: forming an alternating stack of insulating layersand sacrificial material layers that alternate along the verticaldirection over the top surface of the substrate, wherein a set of allinsulating layers within the alternating stack comprises a verticallyalternating sequence of the channel-level insulating layers andinter-transistor-level insulating layers; and replacing the sacrificialmaterial layers with replacement material layers, each of thereplacement material layers including a respective metallic materiallayer, wherein: each sacrificial material layer other than a bottommostone of the sacrificial material layers and other than a topmost one ofthe sacrificial material layers contacts a respective overlying one ofthe insulating layers and a respective underlying one of the insulatinglayers; each of the replacement material layers includes a respectivesource layer containing one of the source regions or a respective drainlayer containing one of the drain regions.
 17. The method of claim 16,wherein: the source layers and the drain layers are formed on theferroelectric gate dielectric layer; and the metallic material layersare formed on a sidewall of a respective one of the source layers andthe drain layers, and are laterally spaced from the ferroelectric gatedielectric layer.
 18. The method of claim 15, further comprising:forming doped semiconductor layers over the substrate such that eachdoped semiconductor layer is formed between, and contacts, a verticallyneighboring pair of one of the channel-level insulating layers and oneof the sacrificial material layers, wherein each of the dopedsemiconductor layers comprises a respective source layer containing oneof the source regions or a respective drain layer containing one of thedrain regions; and replacing the sacrificial material layers withmetallic material layers that contacts a horizontal surface of arespective one of doped semiconductor layers.
 19. The method of claim18, wherein the metallic material layers are formed on a sidewall of theferroelectric gate dielectric layer.
 20. The method of claim 15, furthercomprising forming a plurality of memory openings vertically extendingthrough the alternating stack, wherein the ferroelectric gate dielectriclayer, the bidirectional selector material layer, and the gate electrodeare located within a memory opening of the plurality of memory openings.